In the field of semiconductor fabrication, it is often advantageous to perform inprocess measurements when fabricating a wafer. A sensor such as a interferometer, an optical emission sensor, a pyrometer, or an infrared monitor is used in the reaction chamber to provide feedback on various process parameters. The center of the wafer is often the optimum or preferred location for sensing changes caused by etch or deposition processes. In many processes, the reactant gases which effect the change in the semiconductor surface must be also aimed at the center of the wafer.
One problem, which exists with the measurement of processes using equipment such as advanced vacuum processors (AVP) as shown in FIG. 1 of U.S. Pat. No. 4,842,686, for example, is that changes in films at the center of the wafer may be obscured from the observation sensor by the necessary positioning of a tube for the passage of discharged gases in a centro symmetric fashion. In addition, certain monitoring processes, such as interferometry, are optimized when the sensor is positioned normal to the wafer surface.
Moreover, during deposition processes, when the window is placed off-center, for example upon the plate of the lower surface defining the reactor volume, deposition may occur on the window as a result of indiscriminate deposition throughout the reaction chamber. Unwanted deposition is also a problem in many important etch processes which include deposition of some components as part of their mechanism for obtaining selectivity.
The deposition of a material on the window is disadvantageous for observations based on optical emissions from the gas because the surface film may absorb a varying portion of the emitted radiation. It is also disadvantageous for interferometric measurement of surface film thicknesses, since the formation or alteration of a thin film on the window surface will contribute its own part to the total interference signal observed, and thus make the observed interference pattern difficult to interpret.
In past configurations, an additional gas, such as helium, has been purged past the sensor window to help prevent unwanted disposition. This solution is inadequate, however, because it does not account for the problem of the reactant gas tube obscuring the sensor or the desire to perform measurements at the center of the wafer.
Accordingly, improvements which overcome any or all of the problems are presently desirable.